In order for an orthodontist to make an orthodontic retainer appliance, a mold must be made which is an exact impression of the individual patient's teeth and plate structure. Generally, a mixture of gelatin and plaster of paris is used which is in a fairly springy rubber-like configuration. This impression is then allowed to harden. This mold is then filled with an artificial stone and allowed to set up. This liquid material is generally a mixture of a monomer and polymer which must be evenly blended before and during the setting up period to assure a smooth, uniform and bubble-free product.
Previously, the typical process has been for the orthodonist or technician to manually rock the model and components back and forth gently during the setting up period, to assure an even distribution across the surface of the model, so that a fairly uniform thickness is obtained as the retainer sets. Approximately 5 to 10 minutes is required for the completion of the setting-up process by this manual procedure and it is usually only possible to prepare one retainer at a time. Following initial set, the model with the plastic resin is placed in a warm water bath and put under approximately 30 lbs. pressure.